In recent years, with the increase in population of old people, patients of senile cataract have increased. In aphakic eye patients whose crystalline lens(es) has (have) been enucleated by operation of cataract, the correction for eyesight is generally effected by the use of any of spectacles, contact lenses and intraocular lenses.
In effecting the above eyesight correction by spectacles, the lenses for correction are thick convex lenses having an unattractive appearance, and the image formed on the retina is larger than that in normal eyes. Accordingly, particularly when one eye is aphakic, there arises aniseikonia, thus making larger the fatigue of optic nerve and cranial nerves and giving a big burden to the spectacles users.
Hence, in recent years, there have been developed contact lenses which are superior in oxygen supply to the cornea and which give a smaller burden to the cornea even when worn for a long time, thus providing a very effective means for eyesight correction in aphakic eye patients.
Further, there have been developed intraocular lenses which are implanted into eyes and accordingly require no detachment, and these lenses are gaining acceptance in the market. The intraocular lenses, as compared with the above mentioned spectacles, etc., have many advantages and are expected to further spread in the future.
Incidentally, many of the aphakic eye patients after cataract operation complain of glare, a difference of color vision, etc. In particular, cyanopsia in which objects look bluish is known generally. Cyanopsia is a disease in which, owing to the removal of the crystalline lens(es) originally having an yellow to yellowish brown color, a blue light (a complementary color to an yellow to yellowish brown color) reaches the retina without being weakened and consequently objects look more blue than in normal eyes. Even when artificial intraocular lenses of conventional type have been implanted into eyes, a blue light reaches the retina without being weakened, in the case of eyes implanted with posterior chamber intraocular lenses made of polymethyl methacrylate (PMMA). Consequently there is seen reduction in distinguishability between blue to bluish violet colors, as compared with the case of phakic eyes.
Hence, intraocular lenses capable of correcting cyanopsia are desired, and U.V. absorber-containing intraocular lenses have been commercialized in order to allow the lenses to have a light absorption characteristic close to that originally possessed by human crystalline lenses. However, U.V. absorbers show a very low absorption in a visible light region and is unable to absorb a visible blue light, though they can absorb an U.V. light of 400 nm or less regarded to be a harmful wavelength; therefore, with the U.V. absorber-containing intraocular lenses, the correction of cyanopsia is incomplete. When a large amount of an U.V. absorber is used in an intraocular lens in order to allow the lens to have an increased absorption in a visible light region, other properties of lens material are decreased or worsened in many cases, which is not preferable.
Incidentally, a so-called prepolymer process is known as a process for producing the U.V. absorber-containing intraocular lens mentioned above. In this prepolymer process, a monomer solution comprising a monomer capable of forming a transparent lens material upon polymerization, an U.V. absorber and a polymerization initiator is introduced into a reactor and then heated for a given length of time at a given temperature to obtain a prepolymer of high viscosity; thereafter, the prepolymer is filtered through a filter, casted into a cell constituted by, for example, two glass plates and a gasket, and then further heated for a given length of time at a given temperature to obtain a transparent lens material.
This prepolymer process has such advantages that the prepolyemr casted into the cell scarcely leaks out therefrom because of its high viscosity and that the shrinkage degree in the step of obtaining a transparent lens material from the prepolymer is small, enabling the production of a transparent lens material having a desired shape. On the other hand, the prepolymer process has problems such as (i) polymerization is effected in two steps thereby making the operation complicated, (ii) the control of the polymerization degree and viscosity of the prepolymer obtained in the first polymerization step is difficult; for example, when the polymerization degree is high and resultantly the prepolyemr viscosity is high, the filtration treatment of the prepolymer conducted before its casting into the cell is difficult (the filtration treatment of the prepolymer of high viscosity is extremely difficult particularly when there is used, for example, a filter of 0.2.mu. in pore diameter in order to remove not only dust but also bacteria), and (iii) when a crosslinking monomer is used in order to allow the polymer obtained to have a hardness, etc., an insoluble polymer is formed in the step of obtaining a prepolymer and the filtration treatment becomes difficult also in this case, and further an insoluble polyemr is formed even in the step of polymer production after the filtration treatment and the polymer obtained (an intraocular lens material) becomes non-uniform.
Hence, an object of the present invention is to provide a process for producing a cyanopsia-correctable intraocular lens, which lens is able to eliminate the drawbacks of conventional intraocular lenses containing an U.V. absorber and can effectively correct the cyanopsia arising in aphakic eye patients after operation of cataract.
Further, another object of the present invention is to provide a process for producing a cyanopsia-correctable intraocular lens, which process is able to obtain smoothly without conducting a complicated operation, an intraocular lens capable of effectively correcting the above cyanopsia.